When screen printing with a printing machine, for example when printing banknotes by this method, use is made of one or more hollow cylinders, whose lateral surface consists of a mesh of natural or synthetic fibers. A varnish is used to block up those holes of the mesh which are not part of the image to be printed. A printing ink is fed into the inside of the cylinder and a doctor blade contacts the inside surface of the mesh and pushes this ink through the open holes of the mesh. Usually, when the screen printing cylinder is placed on the machine, one person stands at one end of the screen and inserts the doctor blade into the body of the cylinder. Another person stands at the other end of the cylinder and catches the free end of the doctor blade and with a combined action the two people position the blade in its bearings. To remove the blade, two people carry out the process in reverse. Consequently, both placement and removal of the doctor blade require two people to be present. There is also the risk of the blade touching the mesh during these operations and destroying it. During removal the person pulling the doctor blade towards himself is obliged to pick it up at an intermediate point and inevitably gets ink on himself.
U.S. Pat. No. 5,592,313 provides two ways of avoiding these operations. One way is to have a rail attached to the doctor blade. The rail is engaged in a track situated at one end of the cylinder. The rail/blade assembly is moved parallel to the cylinder axis until it reaches a second track at the other end of the cylinder. The rail/blade assembly is then laid by a vertical movement on two bearings which may pivot about a vertical axis or may be vertically movable. Another way is for the bearings to be in the form of two tracks that move vertically when placing the doctor blade in the working position.
EP-A-0 463 699 also provides a way of placing and removing very long doctor blades. A mechanism of gears, belts and cranks is used to place and remove an assembly comprising the blade, a moving carriage and a guide. The fixing means (bearings) of the blade pivot about the blade. The blade turns on its axis to position the angled inlet of the feeder pipe of the doctor blade.